On the PHPDelusions.com site there's a post that compares the functionality of mysqli to PDO and looks at the differences in their overall usability.

By no means I am going to say that mysqli is worse than PDO. Mysqli is an excellent extension, with many specific features. But it's just not intended to be used directly. To make it usable, one have to always wrap it into a helper library, to reduce the enormous amount of code that otherwise have to be written by hand.

[...] But for the average PHP/MySQL user, standard APIs are the only known methods for database interaction. Thus they tend to use both extensions right in the application code, without any intermediate wrapper around. For such a use PDO is an indisputable winner, and I'll show you why.

The post then breaks it down into sections comparing the functionality between the two database access methods:

• Named placeholders
• General inconvenience in binding
• Getting single column value
• Getting multiple rows
• Binding unknown number of parameters
• Compatibility

Of course, all the inconveniences above could be overcame by a good wrapper. This is why if you choose mysqli, you definitely have to use one.